Design Development and Test of the RIT-ÁX Mini Ion Engine System

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The work presented in this publication has been performed under a contract granted by the European Space Agency ESA in the frame of the GSTP programs. Under lead of Astrium Space Transportation, Business Line "Equipment and Propulsion" a study and development team has been formed bringing together the competences in all relevant fields. Astrium is a well known system house with a long term heritage in electric propulsion. For more than 4 decades a fruitful co-operation with Giessen University has been existing. The Radio Frequency principle was invented in Giessen in the early sixties of the last century by professor Horst.W. Loeb, later-on a whole family of ion engine prototypes was developed under his lead [7][8] [9]. During the last years Giessen University has focused its activities on micro-propulsion [1] [3]. The successful work that had been performed [10] was the basis for the activities described here. Naturally, Giessen University is a strong partner in this project. Giessen University provides knowledge of thruster physics, scaling and layout. Moreover Giessen provides test services for development and endurance testing. Another experienced long-term partner in RIT development is the Institute for Surface Modification (IOM) Leipzig. The IOM has the lead for ion optics and lifetime analysis. Although hardware activities are limited to the ion engine itself the RIT-µX project comprises system engineering too. An essential part for any ion engine is the required power supply and control logic (PSCU). Elaboration of the electronics system is performed by Astrium Satellites GmbH (Friedrichshafen). Design and layout of the flow management system is supported by University of Stuttgart, Institute for Space Systems (IRS). This publication is focused on the functional test results obtained during the project. A more detailed description of the potential system has been yet published [11]. II. Typical low thrust missions Reducing the altitude of earth observing satellites in low earth orbits evidently increases the resolution of the data from the measuring instrumentation. On the other hand the atmosphere's influence on the spacecraft increases. A compensation of the atmospheric drag becomes mandatory. This is challenging for the propulsion system, as high thrust controllability and resolution is required combined with sufficient total impulse. Moreover the high amount of atomic Oxygen requires a robustness of the engine against this aggressive element. Concerns with respect to any technology which requires (hot) cathodes exists. The installation of formation flying satellites forming one large virtual instrument offering a significant higher resolution than any large single instrument is considered as a break through for several types of (deep) space observation missions. Such experiments will deliver important impacts for fundamental physics and our understanding of the universe. For these formations again the controllability and the thrust resolution are from importance. High specific impulse and total impulse are highly desirable to maintain the experiments over years. Ultra fine thrust control is required for some missions dealing with the measurement of gravity waves. Goal is a propulsion system with a thrust starting with zero to some tenth (hundreds) of micro Newtons. As these experiments will take place far from Earth, for example at the Lagrangian point L2 also a higher thrust in the range of some milli Newtons for the cruise phase to the destination is required. III. The RIT-µX Ion Thruster Technology A. RIT-µX Basics Heart of the new RIT-µX small ion propulsion system is the radio frequency ion thruster RIT-µX with the unique electrodeless ionization of the propellant by electromagnetic waves. RF-ionization is known as a very effective way to ionise neutral gases. The implementation of this ionisation principle is very simple as merely two components are necessary: An ionisation chamber made of an isolating material and an RF-coil which surrounds this chamber. No other parts are required inside or outside the ioniser with respect to ionisation of the propellant! This makes the overall concept very simple, robust and erosion free. If an RF-current is applied to the the thrusters RF-coil a primary axial magnetic field is induced inside the ioniser. This field generates a secondary circular electric field (E). Whereas the effect of this electro-magnetic field on neutrals or ions is negligible, free electrons gain sufficient energy for impact ionisation of the propellant: The propellant is set into the plasma state. Once the ionisation process is initially triggered the process is self-sustaining. All electrons required for a steady state operation are generated in the discharge itself. There is no need for an additional electron source, e.g. a main cathode, inside the ionisation chamber. 2 The 31st International Electric Propulsion Conference, University of Michigan, USA September 20 – 24, 2009
In ion thruster technology the rf-principle is unique, but there exists a broad field of applications where rfionization is used. It is employed in plasma- and ion sources for material processing (solid state physics, semiconductors, plasma chemistry…) as well as for neutral injector sources for fusion plasma heating. So the physics behind are well understood. Some special properties, which also apply for ion thrusters, make the RFplasma very favourable for propulsion purposes and especially for micro propulsion. B. Physical Background High Discharge Efficiency: The plasma generation by RF is one of the most efficient methods to set a gas in the ionized plasma state. The efficiency is so high that no additional support, like external magnetic fields for plasma confinement is required. RF-thrusters work without permanent magnets or additional solenoid. Low Electron Temperature: [5][6]The plasma’s electron temperature is comparably lower than for Kaufman thrusters. Therefore the potential drop, which in a first order is directly proportional to the electron temperature, between the quasi neutral inner plasma and its surroundings remains below the sputter thresholds of the materials used for the ionisation chamber and the screen grid. Neither the plasma sided grid nor the ionisation chamber are subject to any kind of erosion processes. Beam Neutralisation Ion Production Ion Acceleration Acceleration RIT-µX Ion Thruster A = Acceleration Grid S = Screen Grid Io = Ionisation Chamber Neut = Neutraliser Ins = Gas Inlet and isolator Rf = Radio Frequency Generator Figure 1 Operating Principle of an RF-Ion Thruster and RIT-µX elegant breadboard. Low amount of multiply charged ions: [5] The power to thrust ratio of any gridded ion engine is increased, if multiple charged ions occur in the ion beam. A low electron temperature means a very small amount of multiple charged ions in the ioniser and the ion beam. The low amount (typ.
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Design Development and Test of the RIT-ÁX Mini Ion Engine System

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